This invention relates generally to airborne weather radar systems and, more particularly, to systems of the type described with aircraft altitude and antenna tilt angle dependent sensitivity time control for normalizing return signals from a target at high as well as low altitudes.
For airborne weather radar systems, the level of return signals from a target is a function of target range. As the range increases, the level of the return signals is reduced. Sensitivity time control (STC) is a well known means for controlling the radar system receiver gain as a function of time, which is equivalent to range, in order to normalize the level of return signals from targets at different ranges.
For airborne weather radar systems with beam filling targets, the return signal levels are proportional to the inverse of the range squared. As a result, normally six dB per octave has been an accepted and specified STC slope. In this regard, reference is made to the publication RTCA DO-173, Minimum Operational Performance Standards For Airborne Weather And Ground Mapping Pulse Radars.
Weather cells, i.e. thunderstorms and the like, have high reflectivity at low altitudes due to rain droplets in the weather cells resulting in high return signal levels. At higher altitudes, the water content in the weather cells is in the form of ice with much lower reflectivity resulting in low return signal levels. The crossover altitude between high and low reflectivity has been found to be within twenty and twenty-five thousand feet. When an aircraft is flying above these altitudes, the radar beam reacts to both low and high reflectivity of the weather cells. As a result, the return signal levels do not follow the aforementioned nominal six dB per octave slope. Thus, if an airborne weather radar system uses the nominal six dB per octave slope for STC, the system excessively attenuates return signals from close-in (high altitude) targets. This problem has been analyzed and described in an article entitled "Comparison of Airborne and Ground Based Weather Radars" by Lester P. Merritt, published in Dec., 1969 by the National Severe Storms Laboratory, ESSA, U.S. Department of Commerce. The present invention obviates this problem by normalizing return signal levels for high as well as low aircraft altitudes.